1. Field of the Invention
This invention relates to manned and unmanned airships and other buoyant and semi-buoyant vehicles, and more particularly relates to the use of multiple fans to provide differential thrust for the controlled maneuver thereof.
2. Description of the Related Art
Historically, airships were only practically rendered steerable, a.k.a. dirigible, with the introduction of proportionately large aerodynamic control surfaces. Most often taking the form of tailfins, thus providing an inherent stabilizing force to the flight path, such control surfaces were effective to maneuver airships provided that sufficient airspeed was attained and the presence of cross winds or vertical drafts were limited. However at slow speeds, the maneuverability attained by this method was severely degraded, and the control surfaces became a significant liability in the presence of strong winds perpendicular to the long axis of the airship. Furthermore, according to Durand, tail fins often accounted for between 18.4% and 26.4% of the combined drag forces of the hull and tail fins on conventional rigid airships. While many modern airships, including proposed and actualized rigid, semi-rigid, and non-rigid types use vectorable thrusters of both ducted fan and unshrouded propeller types, these are generally used in conjunction with tail fins. Furthermore, the means to pivot the vectorable thrusters incurs penalties in terms of weight, cost, and complexity. Pavlecka informs us of the benefits to be gained from eliminating tail fins from airship design and proposes the use of internally ducted, stationary bow and stern thrusters for both directional control and lift augmentation purposes. However, as the thrusters of his design are always arranged orthogonally to the axis of the airship extending from bow to stern, they are not capable of contributing to the airship's forward momentum, and must always serve in conjunction with other prime movers to achieve forward progress or maneuvers with a forward component.